1. Field of the Invention
This invention relates to a photovoltaic cell. More particularly, the invention relates to a high-performance photovoltaic cell which can be reliably used for a long time and which can be subjected to mass production at a low cost.
2. Description of the Related Art p U.S. Pat. No. 4,419,533 discloses a photovoltaic cell in which the surface of a metal layer has a textured structure and in which a back reflecting layer having a transparent layer formed thereon is provided. According to such a configuration, solar light which has not been absorbed by semiconductor layers, comprising thin-film semiconductors, is returned to the semiconductor layers to effectively utilize the incident light. According to the textured structure of the surface of the metal layer and/or the transparent layer, the length of the optical path of reflected light is lengthened, thereby contributing to an increase in the short-circuit current density.
FIGS. 3(a) and 3(b) are graphs illustrating a state in which the absorption of incident light increases as a result of adoption of a back reflecting layer having a textured structure. FIG. 3(a) illustrates the spectral sensitivity of an a(amorphous)-SiGe solar cell in which a specular silver layer is used as a metal layer serving as a back reflecting layer. FIG. 3(b) illustrates the spectral sensitivity when a textured structure silver layer is used instead of the specular silver layer shown in FIG. 3(a). In the solar cell shown in FIG. 3(a), light in the wavelength regions of 700 nm, 780 nm and 850 nm is not effectively utilized in the semiconductor layers comprising an a-SiGe semiconductor. On the other hand, in the solar cell shown in FIG. 3(b), a drop in the spectral sensitivity (Q-value) is hardly observed in these wavelength regions. Accordingly, it can be understood that the use of a back reflecting layer having a high reflectivity for light of these wavelength regions is effective for increasing the conversion efficiency.
FIG. 2 illustrates the results of studies on the reflectivity for Ag, Al, Cu and Ni monolayer films having a thickness of 200 nm. It can be understood from FIG. 2 that while silver and copper show high reflectivity values at the entire wavelength region of 700-1,000 nm required for thin-film semiconductors used in the present invention, aluminum has a minimum value in the vicinity of the wavelength of 800 nm, and nickel has low reflectivity values in the entire wavelength region. Accordingly, silver and copper are suitable as a material for forming a metal layer serving as a back reflecting layer.
Although the use of silver or copper having a superior reflectivity value as a metal for the back reflecting layer is very advantageous for obtaining a solar cell having a high conversion efficiency, these metals, particularly, silver, are known as metals causing electrochemical migration.
Electrochemical migration (hereinafter abbreviated as "migration") is a phenomenon in which, when a metal in the form of a foil, a plated substance, a paste or the like, is used in high humidity while contacting a hygroscopic or hydrophilic insulator in a state in which a DC voltage is applied, the metal grows in the form of a dendrite or penetrates the surface or the inside of the insulator due to electrolysis to form a conductive path. There have been reported that for some metals, conditions other than the ones described above are required. For example, migration occurs in the presence of distilled water and an electric field for silver (Ag), copper (Cu), lead (Pb) and the like (the growth rate of dendrite crystals of Ag is particularly high). For gold (Au), palladium (Pd),indium (In) and the like, migration occurs when halogen ions are present. On the other hand, migration does not occur under the above-described conditions for aluminum (Al), nickel (Ni), iron (Fe) and the like.
In solar cells which will be used in various environments, short circuiting between electrodes due to the above-described migration is a problem during the use of a long time period. For example, consider a case in which solar cells practically used outdoors are in an environment of high temperature and high humidity. In general, since the output voltage of a single solar cell is low, a plurality of submodules (each comprising the above-described thin-film-semiconductor solar cells assembled in the form of a module) are used by being connected in series. When such solar cells are partly covered with fallen leaves or the like, the output currents of the submodules of the covered portion become much smaller than those of other submodules, thereby substantially increasing the internal impedance. As a result, the output voltages of the other submodules are applied in a reverse direction. That is, the conditions for generating migration of high temperature, high humidity and the application of an inverse bias are realized, thereby causing short circuit between electrodes and damage of the submodules. This a technical problem particularly to be solved when using Ag or Cu having a high reflectivity suitable for the back reflecting layer.
In order to solve such a problem, a metal having better migration resistance, i.e., Al, Ni or the like, may be used instead of Ag or Cu. However, when using Al, Ni or the like, although the migration problem is lessened, the photoelectric conversion efficiency decreases because the reflectivity of such a metal is lower than that of Ag or Cu. Particularly, a portion where the reflectivity drops in the vicinity of the wavelength of 800 nm is present for Al.
Japanese Patent Laid-Open Application (Kokai) No. 3-166770 (1991) describes the provision of a diffusion prevention layer comprising copper oxide in order to prevent diffusion of copper used for the back reflecting layer into the semiconductor layers. This approach, however, requires an additional process of oxidizing the surface of copper.